Two hundred corpses, including those of people believed to have been executed by the Islamic State group, were found near the Syrian city of Raqa, a local official and a war monitor said Wednesday.

The mass grave contained the bodies of five middle-aged men in orange jumpsuits of the kind typically worn by IS hostages, Yasser al-Khamees and the Britain-based Syrian Observatory for Human Rights said.

"They were shackled and shot in the head," said Khamees, who heads a team of first responders.

They were believed to have been killed more than two years ago, he said, adding that his team was not immediately able to identify them.

The grave also included the bodies of three women who were believed to have been stoned to death, Khamees and the Observatory said.

"Their skulls were severely fractured and displayed signs of stoning," the local official added.

The digger said his team first discovered the mass grave early last month on the southern edges of Raqa, IS's former Syria capital.

As many as 800 people could be buried there in total, he said.

Its discovery could help identify even more of the several thousand people whose fates remain unknown, including foreigners imprisoned by IS.

IS took full control of the city of Raqa in early 2014 and made it the de facto Syrian capital of its infamous cross-border "caliphate".

US-backed forces ousted IS from the devastated city in October 2017, leaving the Raqa Civil Council (RCC) to run it.

The RCC has been retrieving bodies from the rubble across Raqa, left in ruins by the months-long assault to oust IS.

In February, an exhumation team uncovered a mass grave holding an estimated 3,500 people in Raqa's Al-Fukheikha agricultural suburb -- the largest to date.

Several other mass graves have already been identified around the city, including one in the neighbourhood known as "Panorama," from which more than 900 bodies were exhumed.